5,239 research outputs found
The Net Exchange Between Terrestrial Ecosystems and the Atmosphere as a Result of Changes in Land Use
The general purpose of this research was to improve and update (to 1990) estimates of the net flux of carbon between the world's terrestrial ecosystems and the atmosphere from changes in land use (e.g., deforestation and reforestation). The estimates are important for understanding the global carbon cycle, and for predicting future concentrations of atmospheric CO2 that will result from emissions. The emphasis of the first year's research was on the northern temperate zone and boreal forests, where the greatest discrepancy exists between estimates of flux. Forest inventories suggest net sinks of 0.6 PgC/yr; inversion analyses based on atmospheric data and models suggest much larger sinks 2-3.6 PgC/yr (e.g., Tans et al. 1990, Ciais et al. 1995). The work carried out with this grant calculated the flux attributable to changes in land use. The estimated flux was somewhat smaller than the flux calculated from inventory data suggesting that environmental changes have led to a small accumulation of carbon in forests that exceeds the accumulation expected from past rates of harvest. Two publications have described these results (Houghton 1996, 1998). The large difference between these estimates and those obtained with atmospheric data and models remains unexplained. The recent estimate of a 1.7 PgC/yr sink in North America, alone (Fan et al. 1998), is particularly difficult to explain. That part of the sink attributable to land-use change, however, is defined as a result of this grant
Symmetric Skyrmions
We present candidates for the global minimum energy solitons of charge one to
nine in the Skyrme model, generated using sophisticated numerical algorithms.
Assuming the Skyrme model accurately represents the low energy limit of QCD,
these configurations correspond to the classical nuclear ground states of the
light elements. The solitons found are particularly symmetric, for example, the
charge seven skyrmion has icosahedral symmetry, and the shapes are shown to fit
a remarkable sequence defined by a geometric energy minimization (GEM) rule. We
also calculate the energies and sizes to within at least a few percent
accuracy. These calculations provide the basis for a future investigation of
the low energy vibrational modes of skyrmions and hence the possibility of
testing the Skyrme model against experiment.Comment: latex, 9 pages, 1 figure (fig1.gif
Biotic Contributions to the Global Carbon Cycle: The Role of Remote Sensing
The CO2 content of the atmosphere is increasing currently as a result of the combustion of fossil fuels and the oxidation of vegetation and soils associated with changes in the use of land. Prediction of the atmospheric CO2 concentration in the future requires a better understanding of how important these land-use changes are currently and how important they have been in the past. In this paper we present an analysis of past changes in the terrestrial biota and soils of the earth. The analysis is based on rates of forest harvest and regrowth, rates of land conversion to agriculture, and on the changes in biomass and soil carbon that accompany these uses of land. The results of the analysis show that changes in land use have caused a net release of carbon to the atmosphere that until recently was larger than the release from combustion of fossil fuels. There is still a large uncertainty in the analysis, however, largely because of conflicting reports as to the current rate of disappearance of tropical forests. We outline the kinds of information needed to improve the analysis and believe that remote sensing is of use immediately in reducing the range of uncertainty by a factor of two to four
Symmetric Instantons and Skyrme Fields
By explicit construction of the ADHM data, we prove the existence of a charge
seven instanton with icosahedral symmetry. By computing the holonomy of this
instanton we obtain a Skyrme field which approximates the minimal energy charge
seven Skyrmion. We also present a one parameter family of tetrahedrally
symmetric instantons whose holonomy gives a family of Skyrme fields which
models a Skyrmion scattering process, where seven well-separated Skyrmions
collide to form the icosahedrally symmetric Skyrmion.Comment: 22 pages plus 1 figure in GIF forma
Theory of many-fermion systems II: The case of Coulomb interactions
In a recent paper (cond-mat/9703164) a general field-theoretical description
of many-fermion systems with short-ranged interactions has been developed. Here
we extend this theory to the case of disordered electrons interacting via a
Coulomb potential. A detailed discussion is given of the Ward identity that
controls the soft modes in the system, and the generalized nonlinear sigma
model for the Coulombic case is derived and discussed.Comment: 12 pp., REVTeX, no figs, final version as publishe
Basins of attraction of metastable states of the spherical -spin model
We study the basins of attraction of metastable states in the spherical
-spin spin glass model, starting the relaxation dynamics at a given distance
from a thermalized condition. Weighting the initial condition with the
Boltzmann distribution we find a finite size for the basins. On the contrary, a
white weighting of the initial condition implies vanishing basins of
attraction. We make the corresponding of our results with the ones of a
recently constructed effective potential.Comment: LaTeX, 7 pages, 7 eps figure
Theory of Fermion Liquids
We develop a general theory of fermion liquids in spatial dimensions greater
than one. The principal method, bosonization, is applied to the cases of short
and long range longitudinal interactions, and to transverse gauge interactions.
All the correlation functions of the system may be obtained with the use of a
generating functional. Short-range and Coulomb interactions do not destroy the
Landau Fermi fixed point. Novel fixed points are found, however, in the cases
of a super-long range longitudinal interaction in two dimensions and transverse
gauge interactions in two and three spatial dimensions. We consider in some
detail the 2+1-dimensional problem of a Chern-Simons gauge action combined with
a longitudinal two-body interaction which
controls the density, and hence gauge, fluctuations. For we find that
the gauge interaction is irrelevant and the Landau fixed point is stable, while
for the interaction is relevant and the fixed point cannot be accessed
by bosonization. Of special importance is the case (Coulomb
interaction) which describes the Halperin-Lee-Read theory of the half-filled
Landau level. We obtain the full quasiparticle propagator which is of a
marginal Fermi liquid form. Using Ward Identities, we show that neither the
inclusion of nonlinear terms in the fermion dispersion, nor vertex corrections,
alters our results: the fixed point is accessible by bosonization. As the
two-point fermion Green's function is not gauge invariant, we also investigate
the gauge-invariant density response function. Near momentum , in
addition to the Kohn anomaly we find singular behavior. In Appendices we
present a numerical calculation of the spectral function for a Fermi liquid
with Landau parameter . We also show how Kohn's theorem isComment: Minor corrections and clarifications, and additional references. 30
pages, RevTex 3.0, 3 figures in uuencoded postscript files
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